Removal of iron from aluminum surfaces



United States Patent REMOVAL OF IRON FROM ALUMINUM SURFACES,

Melvin H. Brown, Allegheny Township, Westmorelaud County, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania N0 Drawing. Application April 9, 1958 Serial No. 7275,2125;

3 Claims- (Cl. 41 -4 2).

This invention relates to, and: has as an object, a

convenient, economical treatment for surfaces of aluminum articles to remove iron impurities therefrom. As-

generally used herein, the term aluminum includes both aluminum of various grades and aluminum base;

of the effect which the iron may have upon the material contained or contacted,or even upon the resistance to corrosipn qf the aluminum article itself. For example, a smaltam'ount Off iron onthe surface of the aluminum sometimes has a contaminating effect; in that it is found, to promote undesired polmeriz a tion of certain plastios or to'cat-alyze the decomposition of hydrogen peroxide.

It hasnow-been found that iron impurities mayibp removed from the surfaces of aluminum and aluminum base alloy articles by a method in which the, article is treated-with an aqueous solution containing at least 0.05 percent by weight of; soluble, citrate, ion; and at least 0.005 percent by weight of soluble su lfi te ion, having apH below about- 7-.0. This method is very effective in removing rolled-in iron, or iron left on an aluminum surface after fabrication.

The ions used in the method just described may be introduced as citric and sulferous acids, or as soluble salts of these acids, such as sodium, potassium and ammonium citrates and sulfites, bisulfites, or mixtures thereof. Although, large concentrations of citrate ion may be employed, it is preferred to use a solution containing less than about percent by weight since amounts in excess of this provide little or no additional benefit and unduly increase the cost of the solution. Similarly, large concentrations of sulfite ion may be employed, but little additional benefit is derived. Generally, a solu tion containing less than about 10 percent by weight and preferably less than about 5 percent by weight of sulfite ion is utilized. It is desirable that the combined concentration of sulfite and citrate ions total at least 0.1 percent by weight.

' The pH of the cleaning solution must be less than 7.0. Generally, the preferred solutions have a pH between 1.0 and 5.0. As is well known, the pH of the solution may be adjusted by introducing the ions as the acids and neutralizing with a base whose salts are soluble, such as. ammonium, potassium and sodium hydroxides. To lower the pH, a non-interfering acid such as sulfuric acid may be employed. The more acid solutions, i.e., pH below about 1.0, produce a significant amount of etching which may, however, bedisregarded ment. I

2,901,334 Patented Aug. 25, 1959 "ice having a pH above about 1.Q'1to minimize such etching;

well: s. r m. h s n p nt f: li n aae d am l- The surface may be treated witha solution by im-. me iaa he: e ire a c t e; s u m. o y: fill g the article with thesolution, or by running or spraying the. solut on ovetthe s r e. q n. ny the m ner y which the contact may be effected between the solution, and the? surface. Generally, agitation or. movement of, the solution relative, to the surface of the article short-. ens, thejrequiredg period, of treatment; When the spray] method is employed, there .is an increased. tendency for the, sultite ion to oxidize irpcontactg with air, as. well as to decompose, and release, sulfur dioxide into the airy,

' quite, frequently-reducing the, efieetivenessof the solu-,

tion. A, sipipleybut effective, test for the presence of ironim-v purities on an aluminum surface is, the ferrpxyl test, wherein the surface, is wet with a solution contain ing 3 percent by weight; of; potassiumferricyanide and 3 percent by weight of nitric acid. If any iron is present, a, blue spot will form within a few seconds. The test solutipn should beprepaned dailysince it detepjprates rapidly upon stand-. ing, After sugh a test, the precipitate formed should be removed prior to, the treat rnentof the present invention since i t l a s been found to interfere with the clam s a tia Sean n m tiaamay e fiaqt velv'emplotted, t .a camii sh hi morethe timeoftre tm at. y e methad of. th in e t:- ani ll a with the a. ring of. h r n. G RE-mind i r. he commut g o -s Sc ati and the ni eat effecting contact. Treatment times betweem and 100;. hours nenm. u qramqna howe er; trea a t ll llasa of s ha 2. ,1 Q I are enera ly f i ent: in. he treatm nt. of. hennrma ic en un ered. qq ami a i n. Iffithe olution. a i a e a ains he sur a e. re ment. ften be educed-b 4Qh.% r i flQ unsea Q t hese. r g ired in an unaei t dli mer i i tree? The-r-sp aasu ledi in accordance h the P esen i vention are especially adapted for use at room temperature, i.e., ambient temperatures normally prevailing in commercial and industrial atmospheres such as between about 15 and 40 C. However, these solutions may also be employed with lesser effectiveness at elevated temperatures such as 50 C.

Illustrative of the efiicacy of the present invention is the following table representing the results obtained in the treatment of prepared test specimens containing embedded iron powder, the contamination being considerably more severe than would be generally encountered. The specimens were immersed in cleaning solutions constituted in accordance with this invention and which were maintained at room temperature unless otherwise noted. Referring to the specimens, one group was an aluminum base alloy nominally composed of aluminum, 3.5 percent magnesium and 0.25 percent chromium (designated in the table as A); a second group was oneof the commercial grades of aluminum containing a minimum of 99.60 percent aluminum and a maximum of 0.04 percent manganese (designated as C). The time to clean ance of the impurities generally consisting of silicon and iron (designated as B); the third group was an aluminum base alloy nominally composed of aluminum and 1.2 percent manganese (designated as C). The time to clean reflects the time at which there was no, or an insignificant amount of, iron contamination remaining, as determined by the ferroxyl test.

Table of examples Citrate Sulflte Ion, Ion, pH Metal Hrs. to Remarks Percent Percent Clean .bywt by wt.

A is v O. 05 0. 04 3. 06 g Added as citric acid and sodium bisulfite.

. Added as cltric acld and sodium bisulfite. pH ad usted o 0 55 {g 5 with sulfuric acid. 1 A 8 I. 5. 0 0. 01 1. 8 g }Added as citric acid and sodium suifite.

' 6. 0 3. 8 2. 55 5 }Added as citric acid and sodium bisulfite. w i I A 96 Added as citric acid and sodium bisul flte i pH adjusted- '9 g2 with ammonium hydroxide. A I v Added as citric acid and potassium sulfite. pH adjusted 8 1 i with potassium hydroxide.

, Added as citric acid and sodlumb1sulfite.. pH ad usted 8 5 g with sodium hydroxide. V

A 1 V Added as citric acid and sodium bisulfite. pH ad ustcd 19 55 with sulfuric acid. A 0.5

0. 8 1. 5 3. 1 3 }Added as citric acid and sodium bisulfite.

A s 1. 34 0. 005 2. 18 {B 2 D0.

Added as citric acid and sodium bisulfite. pH ad usted 19 2 5 with sulfuric acid. 1 A 4 Maintained at 50 0. Added as citric acid and sodium 9- 5 {g 2 bisulfite. v

Of the solutions identified in the foregoing table, it is preferred to employ the solution comprised of 0.8 percent byfweight of citrate ion and 1.5 percent by Weight of 'suliite ion,'at room temperature; and this solution may conveniently be made up with citric acid and sodium bisulfites.

' Having thus described the invention, I claim:

' l.. The method of removing iron impurities from a surface of an aluminum article which comprises treating said surface with an aqueous solution consisting essentially'of at least 0.05 percent by weight of soluble citrate ion a'nd at least 0.005 percent by weight of soluble sulfite ion, and having a pH below about 7.0.

' 2. The method in accordance with claim 1 wherein said solution is at a temperature between about 15 and 40 C. V

3. The method of removing iron impurities from a surface of'an aluminum article which comprises treatmg said surface with an aqueous solution consisting es-- sentially of at least 0.05 percent by weight of soluble citrate ion and at least 0.005 percent by weight of solu-j ble sulfite ion, and having a pH of between about 1.0 and 5.0.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFHIATE OF CORRECTION Patent No, 2,901,334 August 25, 1959 Melvin H, Brown It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 64, after "percent' strike out "manganese (designated 3)., The time to clean" and insert instead an copper and 0 01 percent manganese. The bal- Signed and sealed this 9th day of February 1960o (SEAL) Attest:

KARL AXLINE ROBERT c. WATSON Attesting Qfiicer Commissioner of Patents 

1. THE METHOD OF REMOVING IRON IMPURITIES FROM A SURFACE OF AN ALUMINUM ARTICLE WHICH COMPRISES TREATING SAID SURFACE WITH AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF AT LEAST 0.05 PERCENT BY WEIGHT OF SOLUBLE CITRATE TION AND AT LEAST 0.005 PERCENT BY WEIGHT OF SOLUBLE SULFITE ION, AND HAVING A PH BELOW ABOUT 7.0. 